NEW YORK (TheStreet) -- GW Pharmaceuticals (GWPH) stock is spiking 122.59% to $85.61 on heavy volume in early-afternoon trading on Monday after its cannabis-derived drug significantly reduced the amount of seizures in children with a rare form of severe epilepsy in a phase-three trial.

The company's Epidiolex drug lowered the frequency of seizures by 39% in children with a form of epilepsy known as Dravet syndrome, compared to a 13% reduction in children who took a placebo. The difference was statistically significant, TheStreet's Adam Feuerstein wrote this morning. 

The study of the company's Epidiolex drug in treating Dravet syndrome is the first of four final-stage epilepsy trials, with results anticipated this year. 

GW Pharma will use the data to apply for approval of the drug with the FDA, which has already granted Fast Track Designation to accelerate the drug's approval. If approved, Epidiolex would be the first cannabis-based medicine to be endorsed by the FDA.

However, the market for treating children with Dravet syndrome is relatively small, TheStreet's Jim Cramer said on CNBC's Squawk on the Street this morning.

As he explains in the above video, shares have doubled not only because of the epilepsy trial, but also because, as the first pure cannabinoid, it will be prescribed widely for off-label use.

GW Pharma is a U.K.-based biopharmaceutical company.

About 5.95 million shares of GW Pharma have been traded so far today, well above the company's average trading volume of roughly 425,811 shares per day.